1. Field of the Invention
This invention relates to a hollow-fiber filter module for use in the filtration of fluids and a filtration method using the same. More particularly, it relates to a hollow-fiber filter module suitable for such purposes as the treatment of large volumes of water containing suspended matter and the removal of suspended dust from air, as well as a filtration method using a plurality of such modules connected in series.
2. Description of the Prior Art
Hollow-fiber filter membranes have excellent filtering performance and, moreover, enable a large area of filter membrane to be contained in a unit volume. Thus, they have conventionally been used for a wide variety of purposes including the preparation of highly purified drinking water, sterile water for medical use, and the like.
There are two types of hollow-fiber filter modules incorporating such hollow fibers. One type of module filters a fluid from the inside to the outside of the hollow fibers, while the other type filters a fluid from the outside to the inside of the hollow fibers. In modules of the former type, however, the hollow fibers have high resistance to passage of the fluid to be treated. Moreover, suspended matter and other impurities in the fluid to be treated tend to remain in the bore of the hollow fibers and cause clogging thereof, and it is difficult to restore their filtering function. For these reasons, modules of the latter type have generally been used in the treatment of water for various purposes.
On the other hand, there are two known filtration methods: the full flow filtration method in which all of the fluid to be treated is filtered through a module and recovered as a purified fluid, and the cross flow filtration method based on the cross-flow technique. The latter filtration method has the advantage that, even if the fluid to be treated contains suspended matter at high concentrations, it can be filtered at a high rate.
However, most conventional hollow-fiber filter modules have been constructed in such a way that hollow fibers are installed within a straight tube and an inlet and an outlet for the fluid to be treated are provided in the sidewall of the straight tube. This construction is more suitable for use in the full flow filtration method, rather than in the partial filtration method. For example, when a conventional hollow-fiber filter module is used in the treatment of water by partial filtration, the water to be treated forms a unidirectional stream flowing from its inlet toward its outlet and traversing the hollow fibers of the module installed in the water purifier. Accordingly, if the flow velocity of the water to be treated is increased, the hollow fibers undergo an unduly high stress in only one direction and, therefore, tend to become damaged.
Moreover, where it is desired to treat large volumes of water on an industrial scale, the inadequate capacity of a single hollow-fiber filter module makes it necessary to use a plurality of such modules connected in series or in parallel. However, when a plurality of modules connected in series are used, it is unavoidable to employ a connection system in which the water to be treated flows along a complicatedly winding path. This is disadvantageous in that the flow path causes a significant pressure loss of the water to be treated, an unduly high pumping capacity is required, the number of modules usable in connection is limited, and a great loss of energy results.